This invention relates to the field of metal refining. More particularly, this invention relates to a new method and apparatus for regulating the delivery of solid materials from a lance to a pool of metal in a refining vessel, especially a pool of molten iron, during refining. The present invention discloses a process covering the introduction of combustible granulated material via neutral carrier gases into the refining metal.
Recarburization of metal by the addition of carbon from above the pool involves a variety of processes which are well known to those skilled in the art. For example, French patent application No. 2,459,835 discloses a process for blowing calcium carbide on the surface of an iron pool using refining oxygen as a vehicle for the carbide. According to French patent application No. 2,459,835, the carbide transported by the oxygen is surprisingly not oxidized in the jet. Unfortunately, recarburization reactions involving carbide compounds consume large quantities of energy, therefore severly limiting the practical application of the process due to high energy expense.
As an alternative to the energy expensive calcium carbide process described above, the carbon necessary for recarburization may be supplied in the form of anthracite, powdered coke or any other appropriate carbonaceous material. The insufflation (i.e., blowing onto the pool surface) of these other forms of carbon cannot adequately be carried out according to the method described in French patent application No. 2,459,835 without the occurrence of undesirably premature oxidation which could physically damage the operating equipment, such premature oxidation possibly being vigorous and even explosive. Consequently, the carbonaceous material must be injected in conjunction with a noncombustible or neutral carrier gas through utilization of a blowing lance, such as disclosed in Luxembourg application No. 84,433 corresponding to U.S. application Ser. No. 542,429 for "Device for Delivering Gaseous and Solid Materials to a Metal Pool During a Refining Process" filed Oct. 17, 1983, which is assigned to the assignee hereof and is incorporated herein by reference. It is also desirable that a protective screen between the flow of carbonaceous material and the adjacent jets of refining oxygen be provided such as described in said Luxembourg application No. 84,433.
It has been discovered that in order to achieve adequate absorption of the carbonaceous material into the pool, the pool must not only contain large concentrations of oxygen and carbon, but also sufficiently high kinetic energy which will enable the material blowing from the outlet of a lance to penetrate into the pool. This high kinetic energy, which is also required to avoid premature combustion of the carbonaceous material above the pool, is obtained through the mechanism of entraining the carbonaceous material in a powerful neutral gas flow. Unfortunately, this powerful jet of neutral gas has an undesirable cooling effect. This cooling necessitates that the carbonaceous material be blown into the pool in as short a time as possible. This short blowing time, in turn, necessitates a high concentration of solid material in the non-combustible carrier with resultant problems involving blockage in the various carrier gas flow conduits and in the blast pipe of the lance.
The necessity and concern for speed (i.e., minimizing the time) in recarburizing the metal pool is greatly increased if nitrogen is chosen as the carrier gas. It is apparent that the jet of nitrogen will strike the surface of the metallic pool coincident with the refining oxygen making contact therewith. This chemically active area of the pool is at an extremely high temperature. It is well known that the solubility of nitrogen in liquid steel increases with increasing temperature, the resultant costs of later recovering the dissolved nitrogen is therefore greater. Thus, as the time for recarburization increases the amount of nitrogen dissolved in the metal will increase; whereas the time for recarburization decreases, the amount of nitrogen dissolved in the bath will decrease.